The present invention relates to a low IF receiver having particular, but not exclusive, application in selective call receivers, such as wide area digital pagers, and to integrated circuits embodying such receivers.
In order to meet the ever-increasing demands for greater levels of integration with fewer off-chip components, radio receivers are increasingly moving to architectures such as low IF. These receivers, unlike traditional superhet designs, benefit because all the required channel filtering can be performed on chip. However, one feature of this type of receiver is that the image frequency is always relatively close to the wanted signal and is often arranged to be one of the adjacent channels. As a result, separation of the wanted frequency from the image frequency by filtering at RF is impracticable. Instead low IF receivers usually rely on removing the image frequency at base band using cancellation techniques.
One characteristic of a low IF receiver is that it relies on good internal matching in its quadrature architecture to achieve high levels of adjacent channel rejection. This means that in situations where the image channel is large relative to the wanted channel some of the interferer in the image channel can find its way into the wanted signal spectrum. This results in a degradation in bit error rates at the output of the demodulator. Depending on the intended usage of a receiver, in some systems the receiver is never expected to cope with large adjacent channel interferers but in other systems such as paging the adjacent channel specification for the receiver is very stringent.
An object of the present invention is to improve adjacent channel rejection in integrated low IF receivers.
According to a first aspect of the present invention there is provided a quadrature receiver which is adaptable to operate as a low IF receiver for data reception and to operate as a zero IF receiver for image channel measurement.
More particularly, there is provided a low IF receiver comprising first and second mixing means, a source of quadrature related local oscillator signals having first and second outputs coupled to the first and second mixing means, respectively, channel filtering means coupled to outputs of the first and second mixing means, signal derotation means coupled to the channel filtering means, the derotation means having first and second outputs, means for causing the mode of operation of the receiver to be changed from a low IF receiver to a zero IF receiver in order to determine the relative amplitudes of adjacent channel signals by altering the local oscillator frequency to locate the wanted channel substantially symmetrically about DC, widening the bandwidth of the channel filtering means to include the channels adjacent the wanted channel and inserting a DC notch in the signal paths from the first and second mixing means to substantially block the wanted channel, the respective adjacent channel signals appearing on a respective first and second outputs of the derotation means, means for determining which of the signals on the first and second outputs has the smaller relative amplitude and the receiver reverting to its low IF mode by altering the local oscillator frequency appropriately to pass the wanted channel and the smaller of the adjacent channel signals, by restoring the bandwidth of the channel filtering means and by removing the DC notch.
In implementing the receiver made in accordance with the present invention the local oscillator may comprise a frequency synthesizer and in changing the mode of operation from zero IF to low IF only one switching of the synthesizer occurs which reduces the overall level of interference generated and also enables the receiver to be switched to its data receiving mode fairly rapidly.
According to a second aspect of the present invention there is provided an integrated circuit comprising a quadrature receiver which is adaptable to operate as a low IF receiver for data reception and to operate as a zero IF receiver for image channel measurement.
The second aspect of the present invention also provides an integrated circuit comprising a low IF receiver comprising first and second mixing means, means for connecting the first and second mixing means to a source of quadrature related local oscillator signals, channel filtering means coupled to outputs of the first and second mixing means, signal derotation means coupled to the channel filtering means, the derotation means having first and second outputs, means for causing the mode of operation of the receiver to be changed from a low IF receiver to a zero IF receiver in order to determine the relative amplitudes of adjacent channel signals by providing a control signal for altering the local oscillator frequency to locate the wanted channel symmetrically about DC, by widening the bandwidth of the channel filtering means to include the channels adjacent the wanted channel and by inserting a DC notch in the signal paths from the first and second mixing means to substantially block the wanted channel, the respective adjacent channel signals appearing on a respective first and second outputs of the derotation means, means for determining which of the signals on the first and second outputs has the smaller relative amplitude and the receiver reverting to its low IF mode by providing another control signal for altering the local oscillator frequency appropriately to pass the wanted channel and the smaller of the adjacent channel signals, by restoring the bandwidth of the channel filtering means and by removing the DC notch.
According to a third aspect of the present invention there is provided a method of operating a low IF receiver comprising:
(a) reconfiguring the receiver to operate as a zero IF receiver by adjusting a local oscillator frequency so that the wanted channel and unwanted adjacent channels are selected,
(b) determining which of the adjacent channels is the smaller, and
(c) reconfiguring the receiver to operate as a low IF receiver with the local oscillator frequency selected to pass the wanted channel and the smaller of the adjacent channels.
The third aspect of the present invention further provides a method of operating a low IF receiver having frequency translation means including a source of local oscillator signals, channel filtering means coupled to the frequency translation means, signal derotation means coupled to the channel filtering means, the signal derotation means having first and second outputs, the method comprising:
(a) reconfiguring the receiver to operate as a zero IF receiver by altering the local oscillator frequency to locate the wanted channel substantially symmetrically about DC, by widening the bandwidth of the channel filtering means to include channels adjacent the wanted channel and by inserting a DC notch substantially blocking the wanted channel in the outputs of the frequency translation means,
(b) determining the smaller of the signals on the first and second outputs of the signal derotation means, and
(c) reconfiguring the receiver to operate as a low IF receiver by altering the local oscillator frequency such that the frequency translation means passes the wanted channel signal and the smaller of the adjacent channel signals, by restoring the bandwidth of the channel filtering means and by removing the DC notch.
The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein:
FIG. 1 is a graph of frequency versus amplitude showing the presence of a high level interferer on the image frequency,
FIG. 2 is a graph of frequency versus amplitude showing the presence of a low level interferer on the image frequency,
FIG. 3 is a frequency versus amplitude graph illustrating the receiver operating as a zero IF receiver,
FIG. 4 is a block schematic diagram of an embodiment of a low IF receiver made in accordance with the present invention, and
FIG. 5 is a flow chart illustrating the various operations from the point at which the low IF receiver is switched on until it is ready to receive data.